The present invention relates generally to tachometers commonly utilized in electronically controlled servo systems.
Tachometers typically generate a signal (i.e. voltage or frequency) which is substantially proportional to its input shaft rotational velocity. Tachometers are often utilized in electronically controlled servo systems, either as feedback transducers for velocity servomechanisms or as feedback transducers in derivative feedback compensators for position servomechanisms. Such servomechanisms, and their system applications are explained in considerable detail in a book entitled FEEDBACK AND CONTROL SYSTEMS by Di Stefano III, Stubberud and Williams, and published as one of Schaum's Outline Series in Engineering by the McGraw-Hill Book Company of New York.
Conventionally, tachometers include an armature rotating within a fixed permanent magnet field such that voltage generated as a back emf is extracted (i.e., sensed) via a commutator-brush assembly. Unfortunately, the commutator-brush assembly detracts from the performance of such a tachometer because of brush drag and electronic noise as well as other related hysteresis effects. Thus, brushless tachometers utilizing a rotating permanent magnet field and a fixed armature are often used in applications which mandate superior performance. Generally, brushless tachometers are considered superior to brush-type tachometers. However, because brushless tachometers typically have fewer armature windings, they often have greater output voltage ripple.
Accordingly, the present invention is a greatly simplified tachometer which has no armature and therefore no commutator-brush assembly nor solid state switching arrangement. The simplified tachometer is environmentally stable, has a substantially reduced output voltage ripple and can be constructed with extremely low values of rotational inertia if desired.
In particular, the simplified tachometer of the present invention includes an eccentric or other sinusoidally undulating surface formed upon an input shaft whose rotational velocity is to be monitored. Proximity transducers are located within a housing in a quadrature arrangement. The proximity transducers are used to measure co-ordinate displacements x and y of the center of the eccentric with respect to the axis of rotation of the input shaft. The center of the eccentric is offset with respect to the axis of rotation of the input shaft by r, where r is equal to (y.sup.2 +x.sup.2).sup.0.5. Since rotational position .theta. is equal to tan.sup.-1 (y/x), rotational velocity d.theta./dt is equal to [1/(1+(y/x).sup.2)]d(y/x)/dt. This expression can be arithmetically simplified to d.theta./dt=[x dy/dt-y dx/dt]/(y.sup.2 +x.sup.2). Since r.sup.2 =(y.sup.2 +x.sup.2), this can be further simplified to d.theta./dt=[x dy/dt-y dx/dt]/r.sup.2.
No further mathematical manipulation is required because both of the terms x dy/dt and y dx/dt are the product of two terms which preserve their algebraic signs. Thus, the entire expression is a "sign" and "magnitude" correct representation of the rotational speed of the input shaft.
Various other objects and advantages of the present invention will become more apparent to one skilled in the art from reading the following specification taken in conjunction with the appended claims and the following drawings.